1. Technical Field
The present invention relates to the improvement of thin films of iron manganese (FeMn) with respect to both corrosion and wear resistance, and more particularly to the formation of one or more protective layers consisting of nitrides, carbides and oxides.
2. Prior Art
The largest use of magnetic thin films is in memory and storage technologies. Of the thousands of magnetic materials studied for potential device applications, only a relatively few of them have proved to be technologically important. Some of the magnetic materials that have been or which continue to be studied for memory or storage devices, switching elements for logic manipulation, thin-film recording heads, thermomagnetic writing, etc. include iron-nickel alloys, cobalt-nickel, cobalt-phosphorus, copper-nickel-iron, iron oxides, chromium oxide and various ferrites.
Thin films are usually directly deposited on a substrate. The techniques of deposition vary from high-temperature liquid-phase epitaxy such as that used in the fabrication of garnet films, through electroplating, to vapor deposition and sputtering. Applications include magnetic bubble technology, magnetoresist sensors, thin film heads, and recording media.
As described in U.S. Pat. No. 4,103,315, thin films of FeMn, with approximately a 50--50 composition, have been used as an exchange bias layer in a magneto resistive (MR) sensor to reduce the erratic and discontinuous changes in sensitivity and linearity that occur as the domain configuration changes, i.e., the so-called Barkhausen noise.
One of the problems with the use of FeMn is its susceptibility to wear and corrosion. The material is exposed to corrosive environments during the thin film fabrication processes as well as during the operation of the magnetic recording system. The structure of a MR sensor is such as to expose a cross-section of a multilayered thin film stack known as the air bearing surface (ABS). Of these layers, FeMn will predominantly be the most susceptible to corrosion. Coupled with an adverse area ratio, this results in severe galvanic attack of the FeMn layer when the ABS is exposed to a corrosive environment.
While corrosion during a lapping process that exposes the ABS is a concern, of particular interest is the attack that can take place during and after a Reactive Ion Etch (RIE) process that may be used to define rails on the slider ABS. Since the RIE takes place after lapping and provides a very corrosive environment, the exposed FeMn layer is very susceptible to attack. Furthermore, reliability of the device during lifetime exposure in the file environment is an issue. It is therefore of interest to find a means to increase the corrosion resistance and wear resistance of FeMn films.
U.S. Pat. Nos. 4,242,710, 4,618,542 and 4,755,897 describe means of deposition of magnetic alloys which amount to bulk alloying. Through the process of bulk alloying, the bulk properties of the material are not maintained.
A process quite different from bulk alloying is surface alloying or surface modification. Surface alloying allows one to only alter the surface properties of a material, i.e., the bulk properties of the material are maintained. Various surface properties of materials which are influenced by surface composition include among others, wear, corrosion, friction and hardness. Ion implantation is a proven method for surface alloying.
Ion implantation has the advantage of being a finishing process which does not change the appearance of the treated surface. It is a highly controlled process which is limited to the very near surface of the material being treated and thus leaving the bulk properties unaltered. The process of ion implantation generally involves the injection of atoms of a chosen element into a chosen solid material to selected concentrations and depths in order to form an alloy or other product that has a different composition from the original solid and that consequently exhibits different and occasionally highly preferable chemical and physical properties. For example, surface mechanical properties and corrosion resistance may be improved by selectively altering the surface composition of a specimen See Madakson, P. B., J. Appl. Phys., 55, 3308 (1984); and Madakson, P. B., J. Mat. Sci. and Eng., 90, 205 (1987). Such a process is very advantageous for the case of MR heads as corrosion resistance may be imparted with no loss of the bulk magnetic properties.
U.S. Pat. No. 4,772,976 involves the ion implantation of nitrogen ions into the top surface of a thin film as a means of controlling the magnetic properties The ion implantation of nitrogen ions alone into iron and chromium have been reported to increase wear and corrosion resistance (see Yi, L. et al., Vacuum, Vol. 39, pp 263-6 (1989) and Terashima, T. et al., Mater. Sci. and Eng., Vol. 90, pp 229-36 (1987)). Other uses of ion implantation include the implantation of nitrogen, tantalum and inert gas ions with respect to Co-Cr-Mo alloys (U.S. Pat. No. 4,743,308); dual ion implantation of boron, silicon or arsenic ions in silicon for device fabrication (U.S. Pat. No. 4,764,478); the implantation of hafnium and xenon ions to control the corrosion of zirconium alloys (U.S. Pat. No. 4,849,082); and implantation of combinations of nitrogen, carbon, arsenic and chromium ions for improving corrosion and wear resistance of zirconium and zirconium alloys.